Optical fiber gratings are key components in modern telecommunication systems for con- trolling the paths or properties of traveling light. Basically, optical fibers are thin strands of glass capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. They are small diameter waveguides comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction.
Optical gratings are important elements for selectively controlling specific wavelengths of light within optical systems. Such gratings include Bragg gratings, long period gratings and diffraction gratings. Such gratings typically comprise a body of material and a plurality of substantially equally spaced optical grating elements such as index perturbations, slits or grooves.
A typical Bragg grating comprises a length of optical waveguide, such as optical fiber, including a plurality of index perturbations substantially equally spaced along the waveguide length. Thee perturbations selectively reflect light of wavelength .lambda. equal to twice the spacing .LAMBDA. between successive perturbations times the effective refractive index, i.e. .lambda.=2n.sub.eff .LAMBDA., where .lambda. is the vacuum wavelength and n.sub.eff is the effective refractive index of the propagating mode. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings have found use in a variety of applications including filtering, adding and dropping signal channels, stabilization of lasers, reflection of fiber amplifier pump energy, and compensation for waveguide dispersion.
Waveguide Bragg gratings are conveniently fabricated by doping a waveguide core with one or more dopants sensitive to ultraviolet light, e.g., germanium or phosphorous, and exposing the waveguide at spatially periodic intervals to a high intensity ultraviolet light source, e.g., an excimer laser. The ultraviolet light interacts with the photosensitive dopant to produce long-term perturbations in the local index of refraction. The appropriate periodic spacing of perturbations to achieve a conventional grating can be obtained by use of a physical mask, a phase mask, or a pair of interfering beams.
A long-period grating typically comprises a length of optical waveguide wherein a plurality of refractive index perturbations are spaced along the waveguide by a periodic distance .LAMBDA.' which is large compared to the wavelength .lambda. of the transmitted light. In contrast with convention Bragg gratings, long-period gratings use a periodic spacing .LAMBDA.' which is typically at least 10 time larger than the transmitted wavelength, i.e. .LAMBDA.'&gt;10.lambda.. Typically .LAMBDA.' is in the range 15-1500 micrometers, and the width of a perturbation is in the range 1/5.LAMBDA.' to 4/5.LAMBDA.'. In some applications, such as chirped gratings, the spacing .LAMBDA.' can vary along the length of the grating.
Long-period fiber grating devices selectively remove light at specific wavelengths by mode conversion. In contrast with Bragg gratings in which light is reflected and stays in the waveguide core long-period gratings remove light without reflection, as by converting it from a guided mode to a non-guided mode. (A non-guided mode is a mode which is not confined to the core, but rather, is defined by the entire waveguide structure. Often, it is a cladding mode.) The spacing .lambda.' of ti e perturbations is chosen to shift transmitted light in the region of a selected peak wavelength .lambda..sub.p from a guided mode into a nonguided mode, thereby reducing in intensity a band of light centered about the peak wavelength .lambda..sub.p. Alternatively, the spacing .LAMBDA.' can be chosen to shift light from one guided mode to a second guided mode (typically a higher order mode), which is substantially stripped off the fiber to provide a wavelength dependent loss. Such devices are particularly useful for equalizing amplifier gain at different wavelengths.
Diffraction gratings typically comprise reflective surfaces containing a large number of parallel etched lines of substantially equal spacing. Light reflected from the grating at a given angle has different spectral content dependent on the spacing. The spacing in conventional diffraction gratings, and hence the spectral content, is generally fixed.
A common difficulty with all of these grating devices is temperature sensitivity. In Bragg gratings, for example, both n.sub.eff and .LAMBDA. are temperature dependent, with the net temperature dependence for a grating in silica-based fiber exemplarily being about +0.0115 nm/.degree. C. for .lambda.=1550 nm. he temperature-induced shift in the reflection wavelength is primarily due to the change in n.sub.eff with temperature. While such a temperature-induced wavelength shift can be avoided by operating the grating device in a constant temperature environment, this approach requires an oven/refrigerator system. In addition, it requires accurate temperature-control and a continuous use of power.
U.S. Pat. No. 5,042,898 by W. W. Morey et al. discloses apparatus that can provide temperature compensation of a fiber Bragg grating. The apparatus comprises two juxtapose compensating members that differ with respect to the coefficient of thermal expansion (CTE). Both members have a conventional positive CTE. The fiber is rigidly attached to each of the members, with the grating disposed between two attachment points. The apparatus can be designed to apply tensile or compressive stress to the grating. In the latter case the grating is confined in a small tube, exemplarily a silica tube. The prior art designs are typically considerably longer than the grating, e.g. at least 40% longer than the grating device, thus making the temperature compensated package undesirably large. In addition, temperature compensating packages can have a substantial variation of reflection wavelength from one package to another because of the variability in the grating periodicity, minute variations, during package assembly, in the degree of pre-stress applied to each grating, and minute variations in the attachment locations. Accordingly, there is a need for compact temperature compensating fiber grating devices. There is also a need for simple fine adjustment of such device.